1. Field of the Invention
This invention relates to a method of coating particles as well as a method of forming a coating onto an array monolayer of particles, which may be in an adhesive form or previously formed onto a film, and products formed thereby. Such adhesives and films whose uniformly separated particles have now been coated with a conductive interconnection material, such as solder, possess anisotropic conductive pathways and enhanced interconnective capabilities, which are particularly attractive in many commercial applications, such as in the electronics industry.
2. Description of Related Technology
The advent of miniaturized electronics devices has spurred the development of numerous techniques and devices for electrical interconnection, many of which use regular-shaped metallic particles in the assembly process.
The trend towards miniaturized ball grid arrays (xe2x80x9cBGAxe2x80x9d) was initiated in the 1960""s by International Business Machines Corporation (xe2x80x9cIBMxe2x80x9d) through its controlled collapse chip connection process (xe2x80x9cthe C4 processxe2x80x9d) . The C4 process initially used arrays of highly regular copper microspheresxe2x80x94of about 1000 microns in diameterxe2x80x94which were subsequently electrodelessly plated with nickel and then with gold [see U.S. Pat. No. 3,303,393 (Hymes) and P. A. Totta and R. P. Sopher, IBM J. Res. Devel., 226 (1969)].
Microsphere development for BGA and so-called xcexcBGA now seeks such spheres with diameters as small as 100 microns, and accurate control of particle diameter distribution (or dispersity). Surface tension properties of molten metals and alloys have been recognized in the production of regular solid gold and solid solder microspheres with narrow spreads of particle diameters [see K. Tatsumi, et al., Int""l Pack""g Strategy Symp. ""96, 4-1, Kudan Kaikkan Japan (1996)]. Attachment of gold spheres of such diameters to specific locations on substrates often requires attraction of the particles onto a template of undersized holes (relative to the sphere diameter), with subsequent alignment onto the substrate electrode pattern achieved using a vision system. The gold spheres themselves are bonded to the substrate by thermocompression; in contrast, solder particles are attached to previously-fluxed electrode substrates.
The microelectronics industry has moved generally from conventional surface mount technology (xe2x80x9cSMTxe2x80x9d), which uses packaged integrated circuits (xe2x80x9cICxe2x80x9d) with peripheral pin-type connections, to more advanced, yet well-known, technologies, such as BGA, chip scale or chip size packaging (xe2x80x9cCSPxe2x80x9d), and now direct chip attachment (xe2x80x9cDCAxe2x80x9d) or flip chip (xe2x80x9cFCxe2x80x9d) .
CSP uses compact arrays of connection (i.e., inputs/outputs or xe2x80x9ci/oxe2x80x9d) to connect to the IC, BGAs or pin grid arrays (xe2x80x9cPGAxe2x80x9d), depending upon whether the i/o""s have metal spheres or metal pins for connectors. For miniaturized electronics devices, CSP enables higher densities of interconnection in smaller spaces than SMT. The resulting devices can thus be made thinner and more compatible with tape-based manufacturing methods, similar to those developed for tape automated bonding (xe2x80x9cTABxe2x80x9d) technologies.
DCA, or FC, involves direct attachment of the bare IC face down on the substrate. In certain situations, FC uses microscopic solder joints to mediate electrical currents to and from the IC and the board. The solder is reflowed onto metalised bumps already built up on the chip during the fabrication processesxe2x80x94of course, in many instances thermocompressed gold joints may be used in place of the reflowed solder.
Chips attached to ICs by such microscopic joints should maintain or enhance the integrity of their attachment by underfilling the space beneath the chip (which is not occupied by discrete microsolder joints) with a durable adhesive (applied in a liquid form), which is wicked in under the chip and subsequently cured to a durable solid. The so-formed solid serves to bond the chip to the board and protect the microscopic electrical interconnections. Without such an underfill sealant, chips attached by DCA or FC tend to exhibit a higher rate of failure.
Anisotropically conductive adhesives (xe2x80x9cACAxe2x80x9d) and anisotropically conductive adhesive films (xe2x80x9cACFxe2x80x9d) are well-known for their use with electronic device, such as FC, interconnection. [See J. H. Lau, Flip Chip Technologies, McGraw-Hill, NY, Ch. 8-10 (1995), and International Patent Application WO 95/20820, the disclosures of each of which are expressly incorporated herein by reference.] Of course, with ACA/ACF technology underfill sealants are no longer necessary to provide the added benefits of chip bonding, sealing and shock absorbing properties to which reference is made above.
ACAs and ACFs are loaded (at about a 10% wt/wt level) during their formulations or fabrication with conductive fillers. These fillers are typically compliant crosslinked polymer microspheres coated first with nickel and then with gold, or simply with a nickel coating. The latter can further be subjected to electrodeless plating of metals such as copper, palladium, platinum, and the like. These spheres are manufactured commercially in a range of sizes, but within each size range the particle diameter is precise, varying only by fractions of a micron (and accordingly may be considered xe2x80x9cmonodispersexe2x80x9d) . However, such spheres are not available commercially with a further coating of a material such as solder.
Metal-coated polymer spheres offer advantages over solid metal spheres with regard to the control of diameter range and accuracy, as well as the compliant nature of the sphere and control of such compliance.
The ability to control sphere diameter with high precision in these products is a consequence of the way in which they are manufactured. Typically, the spheres are grown from a polymer seed, with the growth reaction being terminated precisely thereby controlling the particle geometries through the propagation of units at the molecular level. Such polymer spheres are then subjected to chemical processes to enhance adhesion of a thin metal seed coating (ordinarily, nickel) deposited by electrodeless plating. Gold is subsequently electrodelessly plated over the nickel seed layer. Electrodeless plating typically deposits metal layers up to about 500 xc3x85 in thickness; it is not a commercially practical method for metal coating deposition of any significant thickness. In addition, to date, it is not believed that solder or resinous materials may be coated onto such spheres in a commercially practical manner by electrodeless techniques.
Other approaches to enhancing polymer particle conductivity include doping their core material with conductive materials, such as silver filled thermoplastic resins. [See U.S. Pat. No. 5,531,942, (Gilleo).] The melting point of metal alloys is much higher than for thermoplastic resins; hence, the thermal performance of electronic devices using solder interconnection materials is superior to electronic devices using metal-filled thermoplastics, particularly in more demanding applications.
Arrays of solder-tipped metal conductors are known. [See U.S. Pat. No. 5,681,647 (Caillat).] However, the system described in the ""647 patent is produced by a complex process involving cathodic sputtering of metals, multilayer film depositions, lithography and electroplating. Such a complex process is likely to be commercially unattractive at least because of the many process steps, and the choice of available chemistry is limited due to thermal sensitivity of the so-described process.
There are, however, disadvantages of metal-coated polymer spheres, including their typical limited current carrying capability by virtue of the small quantity of conductive material present. The metals traditionally used to coat the surface of polymer spheres are not fusible, and hence cannot wet substrate metalisations to make efficient joints. While low current carrying capabilities in particles bearing thin metal coats may be circumvented by increasing particle density, a better solution would be a more substantial layer of conductor overcoating the particle. A coating of a fusible metal alloy, such as solder, would be better still.
There is large commercial demand for metal-coated spheres in electronic applications, such as BGA, as all major microelectronic device manufacturers have some version of BGA-based devices. It is believed that this demand would further increase and extend in the event smaller, monodisperse spheres could be manufactured with a fusible metal surface.
Thus, it would be desirable for a particle to possess the advantages of a metal-coated polymer sphere, as well as conductive properties and other properties such as fusibility of the metal coating.
Despite the state-of-the-art, it would be desirable to provide coated microparticles, as well as enhance the interconnective capability of particles used in ACA/ACF technology.
The present invention provides methods of forming a coating onto a monolayer of particles, and products formed thereby.
This invention also provides methods for forming a coating onto arrayed monolayers of particles, which particle arrays are maintained in place by use of a cured tack layer. These methods employ a curable matrix in which the particles are dispersed and the curable matrix is cured partially through the liquid film to form a thin solid film. The so-formed thin film maintains the particles in place, but does not substantially encase the particles. Accordingly, the particles (now maintained in place) are ripe for coating with an appropriate material of choice.
The present invention also provides methods for producing films from such coated particle-containing curable matrices, where the particles are maintained and contained within an additional film. In these methods, the monolayers of particles are backfilled with a film-forming material which substantially encases and securely maintains in place the particles.
By the methods of the present invention, coated polymer particles of precise diameter are produced by a simple process where the particles are isolated from one another in one substrate plane and form a non-random distribution pattern in that plane. In a particularly desirable aspect of the methods of this invention, wave solder coating of existing metal-coated polymer spheres is achieved in a process which is made possible by the initial separation of the particles in a uniform manner which enables individual particle coating with minimum interparticle bridging by the coating material. In the methods of the invention, desirable topographies may be produced on the particles, such as spikes, in a facile on-line process. Such topographies may be particularly useful in achieving advantageous interconnective properties, which are likely to be commercially desirable in at least the electronics industry.
Accordingly, the present invention is advantageous as it provides methods of coating particles, particularly microparticles, with a material, such as solder, which otherwise would be impractical, leaving the products formed by this invention unattained. Thus, through the material coating methods of this invention, the surface properties of monolayers of such particles are modified, rendering the particles more versatile for existing and new electronic applications, such as in CSP and FC technologies.
That is, this invention is directed to an on-line method of coating the surface of individual microparticles (whose diameters range from about 5 microns up to about 1000 microns) with a variety of coating materials. The thickness and surface topology of the chosen coating material may be controlled using the methods of this invention. Indeed, the methods of this invention may coat particles with a variety of materials in an on-line, rapid, continuous and facile manner.
The invention will be more fully understood by a reading of the xe2x80x9cDetailed Description of the Inventionxe2x80x9d together with the drawing.